SYNTHESIS AND CHARACTERIZATION TECHNIQUES A
3.4 SQUID Magnetometer
Superconducting Quantum Interface Device (SQUID) magnetometer MPMS XL, USA is the most sensitive available device for measuring magnetization at the Department of Engineering Sciences, ngstrom laboratory, Uppsala University, Sweden.
The magnetic properties measurement system MPMS XL is a sophisticated analytical instrument configured specially for the study of the magnetic properties of small samples over a broad range of temperature from 4.2 K to 400 K and magnetic fields from -50 We to +50 KOe. This standard system can measure the magnetic moment of solid, powder
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and liquid samples with a differential sensitivity of 10 9 emu and can handle a maximum signal size of 0.5 cmu.
A general view of the MPMS XL with its system components is shown in Fig. 3.8 SQUID magnetometers are used to characterize materials when the highest detection sensitivity over a broad temperature range and using applied magnetic fields up to several Tesla is needed. Nowadays, this instrument is widely used worldwide in research laboratories. The system is designed to measure the magnetic moment of a sample, from which the magnetization and magnetic susceptibility can be obtained. Therefore, SQUID magnetometers are versatile instruments that perform both, DC and AC magnetic moment measurement. MPMS Multi Vu is the software application controlling the RSO system.
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Fig 3.8 MPMS XL SQUID Magnetometer
The major components of a SQUID magnetometer are: The RSO sample rod consists of one long, graphite sample rod; one short, graphite sample rod; and two centering plugs, superconducting magnet, superconducting detection coil, a SQUID connected to the detection coil, superconducting magnetic shield. Superconducting magnets are solenoid made of superconducting wire which must be kept at liquid helium
Dewar. The uniform magnetic field is produced along the axial cylindrical bore of the coil. The superconducting pick-up coil system, which is configured as a second order gradiometer is placed in the uniform magnetic field region of the solenoidal superconducting magnet. The SQUID device is usually a thin film that functions as an extremely sensitive current to voltage converter.
During an RSO measurement, a shaft encoder on the servo motor tracks the position of the sample. The position is recorded synchronous to the acquisition of the SQUID signal. The data is analyzed by using a nonlinear, least-squares fitting routine to fit the data to an ideal dipole response. The magnetic moment calibration for the MPMS is determined by measuring a palladium standard over a range of magnetic fields and by then adjusting the system calibration factors to obtain the correct moment for the standard. The standard is a right circular cylinder approximately 3 mm in diameter x 3 mm in height. Samples of this size or smaller effectively imitate a point dipole to an accuracy of approximately 0.1%.
Measurements are done in this equipment by moving the samples through the second order gradiometer. Hence, the magnetic moment of the sample induces an electric current in the pick-up coil system. Superconducting magnetic shield is used to shield the SQUID sensor from the fluctuations of the ambient magnetic field of the place where the magnetometer is located and from the large magnetic field produced by the superconducting magnet. It is an important feature of the instrument that one can change the magnetic field either by "oscillate mode" or "no overshoot mode". The oscillate mode is used to minimize the remanent field of the magnet, whenever an accurate value of magnetic field is needed, e.g. in case of zero field cooling. In the hysteresis measurement the no overshoot mode has been selected, in which the field is changed directly from one value to another, and the magnet is returned to its persistent mode.
The accuracy of an RSO sample measurement is determined by how well the sample is centered within the SQUID pickup coils. If the sample is not centered, the coils read only part of the magnetic moment of the sample. Properly centering the sample is particularly important if it will be measured at the maximum slope position. During maximum slope position measurements, MPMS MultiVu cannot use auto tracking or the iterative regression algorithm, which both help keep the sample correctly positioned, even when temperature changes alter the geometry of the sample rod.
The new temperature sweep mode of operation provides MPMSXL users with the ability to take magnetic measurements while sweeping the system temperature at a controlled rate, automatically with no manual intervention. This mode provides a
controlled, monotonic change in temperature during a measurement sequence at rates up to 10 Klmin. Measurements of temperature dependence over large temperature ranges, which previously required time consuming temperature stabilization, can now be made quickly and precisely using temperature sweep mode.
The main components of a SQUID magnetometer are:
Superconducting magnet (that must be acquired together is programmable bipolar power supply);
Superconducting detection coil which is coupled inductively to the sample
A SQUID connected to the detection coil and Superconducting magnetic shield.
High sensitivity is possible because this device responds to a fraction of the flux quantum. The SQUID device is usually a thin film that functions as a extremely sensitive current to voltage converter. A measurement is done in this equipment by moving the sample through the second order gradiometer. Hence the magnetic moment of the sample induces an electric current in the pick-up coil system. A change in the magnetic flux in these coils changes the persistent current in the detection circuit. So, the change in the current in the detection coils produce variation in the SQUID output voltage proportional to the magnetic moment of sample.
The difference between low and high temperature SQUID's and their suitability for specific applications is discussed. Although SQUID electronics have the capability to operate well above 1 MHz, most applications tend to be at lower frequencies.
Specific examples of input circuits, detection coil configuration for different applications and environments along with expected performance.
SQUID measures the magnetic moment by moving the sample through the detection coil. The magnetic moment of the sample creates a flux in the detection coil, which changes with the sample position. The flux is converted to a voltage by the magnetometer and the voltage versus sample position data is further used to extract the magnetic moment. The software package supplied with the MPMS analyzes the voltage versus sample position data and determines a value for the magnetic moment as a result of a fitting procedure between a theoretically expected curve and the experimental data.